Method and device for determining a vehicle inertial position

ABSTRACT

The inertial state can be determined independently of dynamic vehicle movements by measuring the accelerations of the vehicle in the direction of its longitudinal, transverse and vertical axes, by forming the magnitude of an acceleration vector resulting from the three acceleration components and comparing the magnitude of the acceleration vector through threshold decisions with a window that is delimited by a threshold lying above and a threshold lying below gravitational acceleration g. A current course angle of the vehicle with respect to its longitudinal axis and/or the current course angle with respect to its transverse axis is then determined only if the magnitude of the acceleration vector lies inside the window; otherwise, however, the previously determined course angles are retained.

FIELD OF THE INVENTION

The present invention relates to a method and arrangement fordetermining an inertial state of a vehicle.

BACKGROUND INFORMATION

German Patent Application No. 196 09 717 describes an arrangement fordetecting roll-over occurrences in vehicles. In case roll-over of avehicle occurs, all passenger protection devices installed in thevehicle must be triggered promptly, including, for example, roll bars,belt tighteners and various airbags. To enable prompt triggering of allof these protection devices, it must be detected as soon as possiblewhether rotations of the vehicle about its vertical axis, itslongitudinal axis or its transverse axis lead to a roll-over. Incorrectdecisions on a roll-over occurrence must be ruled out insofar aspossible so that the restraint devices are not triggered when, forexample, the vehicle is on a steep slope or undergoes slow rotationaloccurrences during travel through curves. To prevent incorrect decisionsfrom occurring in the roll-over sensing, the inertial state, i.e., theinitial state of the vehicle relative to the earth-based coordinatesystem, must be known. Dynamic vehicle movements such as travel throughcurves or braking or acceleration procedures can have disruptive effectsin determining the inertial state.

SUMMARY OF THE INVENTION

An object of the invention is to specify a method and an arrangement fordetermining the inertial state of a vehicle, disruptive effects due todynamic vehicle movements being ruled out to the greatest possibleextent.

Initially, the accelerations of the vehicle in the direction of itslongitudinal, transverse and vertical axes are measured. Then, themagnitude of an acceleration vector resulting from the threeacceleration components is formed and this magnitude is compared throughthreshold decisions with a window that is delimited by a threshold lyingabove and a threshold lying below the gravitational acceleration. Thecurrent positional angle of the vehicle with respect to its longitudinalaxis and/or the current course angle with respect to its transverse axisis then determined only if the magnitude of the acceleration vector liesinside the window. However, if the magnitude of the acceleration vectorlies outside the window, then a previously determined course angle withrespect to the longitudinal axis and/or course angle with respect to thetransverse axis is retained.

In this method or rather a corresponding arrangement for carrying outthis method, dynamic acceleration components of the vehicle are ruledout with the aid of the window function when determining the inertialstate. The course angles of the inertial state can then be correctlymeasured with the three acceleration sensors if the vehicle is eithernot moving at all or is moving uniformly. If the vehicle is subject todynamic state changes, then no new current course angles are determined;instead, one falls back on previously determined course angles that areuninfluenced by dynamic position changes.

It is advantageous that the upper threshold of the window is about 10%greater and the lower threshold about 10% less than gravitationalacceleration. Interfering quantities in determining the inertial statecan be suppressed even more effectively if the threshold decisions areperformed with respectively two acceleration vectors formed one afteranother in time. By forming the current course angle recursively from acomponent of a previously determined course angle and a component of thecourse angle derived from the currently measured accelerations,short-term disruptions can be suppressed very well in an advantageousmanner.

Roll-over occurrences are very fast state changes of the vehicle thatcan be best detected with rate of rotation measurements. Based on themeasured rates of rotation, the course angles are then derived throughintegration and it is decided based on these course angles whether aroll-over of the vehicle is occurring or not. So that dynamic vehiclemovements uncritical to a roll-over occurrence do not also enter intothe integration of the measured rates of rotation and the resultingcourse angles do not lead to an incorrect decision concerning aroll-over occurrence, it is useful to not begin integration of the ratesof rotation prior to the availability of the inertial course anglesderived according to the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an earth-based and a vehicle-based coordinate system.

FIG. 2 shows a functional diagram for deriving the inertial state of thevehicle according to the present invention.

FIG. 3 shows a window function.

DETAILED DESCRIPTION

In FIG. 1, an earth-based coordinate system x, y, z is sketched which isoriented such that gravitational acceleration g acts in the direction ofthe z axis. Also sketched in FIG. 1 is a vehicle-based coordinate systemx′, y′, z′ of a vehicle which exhibits a certain banking with respect tothe earth-based coordinate system. In the vehicle-based coordinatesystem, x′ is the longitudinal axis, y′ the transverse axis and z′ thevertical axis of the vehicle. In the vehicle, there are accelerationsensors which measure the acceleration components of the vehicle in thedirection of its longitudinal axis x′, its transverse axis y′ and itsvertical axis z′. Gravitational acceleration g is divided among theindividual acceleration components ax′, ay′ and az′ depending on theposition of the vehicle-based coordinate system x′, y′, z′. If, due todynamic vehicle movements, one or more further acceleration componentsare superimposed on gravitational acceleration g, it is no longerpossible to determine error-free based on the measured accelerationcomponents ax′, ay′ and az′ the actual course angles of the vehiclerelative to the earth-based coordinate system. As a general rule, thecourse angle φx, which is the rotational angle of the vehicle-basedcoordinate system about the x axis of the earth-based coordinate system,and the course angle φy, which is the rotational angle of thevehicle-based coordinate system about the y axis of the earth-basedcoordinate system, are needed to characterize the inertial state of thevehicle. As will be explained based on FIG. 1, the two course angles φxand φy can be derived using trigonometric functions from theacceleration components ax′, ay′ and az′ and gravitational accelerationg. If interfering quantities due to dynamic vehicle movements are nowsuperimposed on the acceleration components ax′, ay′, az′, this leads tocorrupted course angles φx and φy of the vehicle.

Based on the functional diagram shown in FIG. 2, a method will now bedescribed for determining, based on the acceleration components ax′, ay′and az′ measured in the vehicle, the course angles φx and φy, which areuninfluenced to the greatest possible extent by interfering quantitiesdue to dynamic vehicle movements. In functional block 1, the measuredacceleration components ax′, ay′ and az′ are subjected to filtering. Thefiltering serves to filter out small disturbances of the individualacceleration components. A suitable filter is, e.g., a median filter orsome other digital filter with a low-pass characteristic. In a medianfilter, each acceleration component ax′, ay′, az′ is sampled over acertain time interval and all of the sampled values are subdivided intoseveral data tupels. For each data tupel, the average sampled value isdetermined. Assuming there are i data tupels from each accelerationcomponent ax′, ay′ and az′, the filtered acceleration components ax′(i),ay′(i) and az′(i) are present at the outputs of the filter 1.

In the second functional block 2, a resulting acceleration vector isformed from the filtered acceleration components ax′(i), ay′(i) andaz′(i) and from this the magnitude:

|a′(i)|={square root over (ax′+L (i+L )² +ay′+L (i+L )² +az′+L (i+L )²+L)}  (1)

In connection block 3, the magnitude of the acceleration vector |a′(i)|is subjected to a threshold decision. As shown in FIG. 3, a windowfunction is involved here. This window has an upper threshold co and alower threshold cu. The upper threshold co is about 10% greater thangravitational acceleration g and the lower threshold cu about 10% lessthan gravitational acceleration g. With this window function, it is thusdetermined whether the magnitude of the acceleration vector is equalmore or less to that of gravitational acceleration g. If, namely, themagnitude of the acceleration vector deviates by a certain degree, whichis specified by the thresholds co and cu, from gravitationalacceleration g, then one must assume that the acceleration componentsax′, ay′ and az′ measured in the vehicle have interfering componentssuperimposed on them due to dynamic vehicle movements. Diagram block 3sets its output signal h to 1 if the magnitude of the accelerationvector lies inside the window and sets its output signal h to 0 if themagnitude of the acceleration vector lies outside the window. One canalso, as is shown in FIG. 3, observe respectively two accelerationvectors present one after another in time on diagram block 3 with regardto their position in the specified window. In other words, only if themagnitudes of the acceleration vectors present at instant i and atinstant i—1 both lie inside the specified window, the signal h is set to1, and otherwise to 0. $\begin{matrix}{h = \left\{ \begin{matrix}{1,} & {{if}\quad {{a^{\prime}(i)}}\quad {and}\quad {{a^{\prime}\left( {i - 1} \right)}}\quad {within}\quad {the}\quad {window}} \\0 & {otherwise}\end{matrix} \right.} & (2)\end{matrix}$

In functional block 4, the course angles φx and φy are derived with theaid of the computational procedure described hereafter from theacceleration components ax′ and ay′. In equation (3), the trigonometricrelationship between the acceleration components ax′, ay′, az′ measuredin the vehicle and the acceleration components with respect to theearth-based coordinate system x, y, z is shown. Since gravitationalacceleration g acts only in the direction of the z axis of theearth-based coordinate system, acceleration ax and ay in the directionof the x axis and the y axis of the earth-based coordinate system are 0.Therefore: $\begin{matrix}{\begin{bmatrix}{a\quad x^{\prime}} \\{a\quad y^{\prime}} \\{a\quad z^{\prime}}\end{bmatrix} = {\begin{bmatrix}{\cos \quad \phi \quad y} & 0 & {{- \sin}\quad \phi \quad y} \\{\sin \quad \phi \quad x\quad \sin \quad \phi \quad y} & {\cos \quad \phi \quad x} & {\sin \quad \phi \quad x\quad \cos \quad \phi \quad y} \\{\cos \quad \phi \quad x\quad \sin \quad \phi \quad y} & {{- \sin}\quad \phi \quad x} & {{\cos \quad \phi \quad x\quad \cos \quad \phi \quad y}\quad}\end{bmatrix}\quad\begin{bmatrix}0 \\0 \\{- g}\end{bmatrix}}} & (3) \\{\begin{bmatrix}{a\quad x^{\prime}} \\{a\quad y^{\prime}} \\{a\quad z^{\prime}}\end{bmatrix} = \begin{bmatrix}{\sin \quad \phi \quad y} \\{{- \sin}\quad \phi \quad x\quad \cos \quad \phi \quad y} \\{{- \cos}\quad \phi \quad x\quad \cos \quad \phi \quad y}\end{bmatrix}} & (4) \\{{\phi \quad y} = {\arcsin \quad \frac{a\quad x^{\prime}}{g}}} & (5) \\{{\phi \quad x} = {{- \arcsin}\quad \frac{a\quad y^{\prime}}{g\quad \cos \quad \phi \quad y}}} & (6)\end{matrix}$

If the window signal h=1, however, errors can still arise when computingthe course angles if, e.g., a resulting vector from interferingaccelerations and the gravitational acceleration coincidentally assumesa magnitude of 1 g. To prevent such an error from occurring, it isuseful to compute each new course angle φx_(new) and φy_(new)recursively from a component of a course angle φx_(old) and φy_(old)computed earlier and a component of the course angles φx and φy derivedfrom the currently measured accelerations:

φx _(new) =c1φx _(old) +c2φx  (7)

φy _(new) =c1φy _(old) +c2φy  (8)

The weighting factors c1 and c2 in the equations (7) and (8) must bedetermined experimentally. They lie between 0 and 1 and have low-passproperties.

In case roll-over occurrences of the vehicle are to be sensed, afunctional block 5 is provided which determines the rotational angle αxand αy of the vehicle about the earth-based x and y axes by integratingmeasured rates of rotation ωx′, ωy′, ωz′ about the longitudinal axis x′,transverse axis y′ and vertical axis z′ of the vehicle. To preventslight dynamic positional changes of the vehicle from also entering intothe integration, the integration is begun with the previously determinedcourse angles αx and αy, the reason being that these course angles αxand αy are largely uninfluenced by disruptive dynamic vehicle movements(e.g., travel through curves, acceleration and braking processes).

What is claimed is:
 1. A method for determining an inertial state of avehicle, comprising the steps of: (a) measuring a first accelerationcomponent of the vehicle in a first direction, a second accelerationcomponent of the vehicle in a second direction and a third accelerationcomponent of the vehicle in a third direction, the first directionextending along a longitudinal axis of the vehicle, the second directionextending along a transverse axis of the vehicle and the third directionextending along a vertical axis of the vehicle; (b) obtaining amagnitude of an acceleration vector of the vehicle as a function of thefirst, second and third acceleration components; (c) determining if themagnitude is within a predetermined threshold range using a thresholddecision arrangement, the predetermined threshold range having an upperthreshold value which is greater than a gravitational acceleration valueand a lower threshold value which is less than the gravitationalacceleration value; (d) only if the magnitude is within thepredetermined threshold range, determining at least one of a firstcurrent course angle of the vehicle with respect to the longitudinalaxis and a second current course angle of the vehicle with respect tothe transverse axis; and (e) if the magnitude is outside of thepredetermined threshold range, retaining at least one of a firstpreviously determined course angle with respect to the longitudinal axisand a second previously determined course angle with respect to thetransverse axis.
 2. The method according to claim 1, wherein the upperthreshold value is approximately 10% greater than the gravitationalacceleration value and the lower threshold value is approximately 10%less than the gravitational acceleration value.
 3. The method accordingto claim 1, wherein step (c) includes the substep of determining if afurther magnitude of a further acceleration vector is within thepredetermined threshold range using the threshold decision arrangement,the further acceleration vector being formed before the accelerationvector, and wherein step (d) includes the substep of determining atleast one of the first current course angle and the second currentcourse angle if the magnitude and the further magnitude are within thepredetermined threshold range.
 4. The method according to claim 1,further comprising the steps of: (f) determining a first correspondingcomponent of the first and second current course angles as a function ofthe first, second and third acceleration components; (g) determining asecond corresponding component of at least one of the first and secondpreviously determined course angles; and (h) recursively determining atleast one of the first and second current course angles as a function ofthe first and second corresponding components.
 5. The method accordingto claim 1, further comprising the step of: (i) determining at least oneof a first rotational angle with respect to the longitudinal axis and asecond rotational angle with respect to the transverse axis as afunction of an integration of at least one measured rate of rotation,wherein the integration is performed using at least one of the firstcurrent course angle, the second current course angle, the firstpreviously determined course angle, and the second previously determinedcourse angle.
 6. An arrangement for determining an inertial state of avehicle, comprising: acceleration sensors measuring a first accelerationcomponent of the vehicle in a first direction, a second accelerationcomponent of the vehicle in a second direction and a third accelerationcomponent of the vehicle in a third direction, the first directionextending along a longitudinal axis of the vehicle, the second directionextending along a transverse axis of the vehicle and the third directionextending along a vertical axis of the vehicle; a first arrangementdetermining a magnitude of an acceleration vector as a function of thefirst, second and third acceleration components; a second arrangementdetermining if the magnitude is within a predetermined threshold range,the predetermined threshold range having an upper threshold value whichis greater than a gravitational acceleration value and a lower thresholdvalue which is less than the gravitational acceleration value; and athird arrangement determining at least one of a first current courseangle of the vehicle with respect to the longitudinal axis and thesecond current course angle of the vehicle with respect to thetransverse axis only if the magnitude is within the predeterminedthreshold range, wherein, if the magnitude is outside of thepredetermined threshold range, then at least one of a first previouslydetermined course angle with respect to the longitudinal axis and asecond previously determined course angle with respect to the transverseaxis is retained.